Yii 1.1.x CDbHttpSession错误：CDbConnection无法打开数据库连接：找不到驱动程序

I'm trying to use CDbHttpSession in Yii 1.1.x but when I change it in my main.php from CHTTPSession I get the following error:

CDbConnection failed to open the DB connection: could not find driver

I have PDO drivers successfully installed on my Linux server and I can connect to the database anyway, I only get this error when I try to use CDbHttpSession. I also allowed the PDO extensions in my php.ini:

extension=pdo.so
extension=pdo_mysql.so

I have php5-mysql installed and I restarted the apache2 of course.

I have the following in my main.php:

'components' => array(

    ...

    'session' => array(
       'class' => 'CDbHttpSession',
    ),

    ...

    'db' => array(
        'connectionString' => 'mysql:host=localhost;dbname=databasename',
        'emulatePrepare' => true,
        'username' => 'username',
        'password' => 'password',
        'charset' => 'utf8', 
        'tablePrefix' => '',
    ),

    ...

),

Thanks in advance for your helping answers.

You need to specify connection ID otherwise Yii tries to use SQLite and you obviously don't have driver for it.

'session' => array(
   'class' => 'CDbHttpSession',
   'connectionID' => 'db',
),